The use of vehicle trim panels, door panels, dashboards, instrument panels etc. with an integrated passenger airbag unit has become well established practice in car manufacturing industry. Structural integrity to the panel is provided by a rigid carrier. A layer of a compressible foam applied on top of the rigid carrier, usually in a thickness of 5-10 mm of a compressible foam, provides a soft tactile feel to the panel and levels uneven areas on the surface of the underlying carrier. The decorative and substantially non-structural skin applied on top of the foam layer usually has a thickness of 1-1.5 mm and is often made from a pliable polyvinyl chloride (PVC), a sprayable urethane elastomer material or a thermoplastic elastomer, thermoplastic olefin or thermoplastic polyurethane.
Polyvinyl chloride (PVC) skins are usually produced using slush molding. A box filled with a particulate PVC compound positioned beneath and locked to a heated mould part, supplies powder to the mould. The mould is repeatedly inverted to melt powder onto the hot mould surface and induce sintering of the particles. After the PVC particles have sintered together, a sheet of plasticized PVC is formed, the sheet or skin is cooled and removed from the mould. Slush molding of PVC compounds is for example disclosed in U.S. Pat. No. 4,562,025. An alternative to slush molding for producing PVC skins includes deep drawing of a flexible PVC foil or sheet into the desired shape.
Such skins when used in vehicle panels must however meet a number of stringent criteria, which relate a.o. to color stability and dimensional stability at high temperature and under prolonged UV exposure at elevated temperature, resistance to a wide variety of chemical compounds such as cleaning agents, human fluids, etc. A major challenge to be met by the decorative skin is that it must provide for a quick opening, to allow a quick and clean opening of the passenger airbag stored underneath the foam layer, through an opening provided in the rigid carrier underneath. To facilitate opening, the skin usually contains a weakening line or tear seam adapted to be torn or fractured by the force of the inflating air bag. In the course of time, the technique of producing such seams has undergone significant evolution, together with the changing design criteria and increasingly stringent safety specifications that are being imposed by car manufacturing industry. Whereas in the past the passenger airbag compartment cover was designed as a separate object that was positioned on top of the airbag compartment, modern car design has evolved towards instrument panels with a smooth, uninterrupted visible surface with built-in airbag compartment. To avoid that the tear seam would become visible in the course of time, the skin needs to possess a good resistance against ageing when exposed to heat and/or UV for long periods of time.
A known technique to produce a skin for a vehicle panel with an air bag tear seam is for example disclosed in U.S. Pat. No. 5,580,083. According to U.S. Pat. No. 5,580,083 a tear seam providing a local weakening in the skin is filled with a filler strip made of the same material as the cover or a thermoplastic material compatible therewith. The filler strip does however not thoroughly bond with the outer skin. Another known technique is disclosed by EP0590779, according to which the tear seam pattern is filled with a thermoplastic of lesser strength material to form an integrally bonded thermoplastic filler strip.
Modern design requirements that the airbag opening should be invisible and resist ageing need however be balanced by the safety criteria imposed by the industry, which require that the skin should allow for a quick and clean opening of the passenger airbag along the weakening line, within milliseconds to permit a fast deployment of the airbag. Another important safety criterion is that the skin-foam-carrier sandwich structure should break along the weakening line without particle shattering when the airbag bursts through the cover, at all operating conditions that might be encountered by the vehicle. Fragments released from the skin, foam or carrier, flying or being projected at high speed towards the passenger are to be kept minimal in all circumstances and should remain within certain limits. These safety criteria must be met by the instrument panel or any other type of airbag containing panel in a vehicle, across a wide temperature range, of at least between −35 and 80° C.
When designing instrument panels and materials for constructing such instrument panels it is a real challenge to provide an optimum compromise between on the one hand design criteria which impose that visibility of the airbag weakening line should be minimal also upon ageing, and airbag performance on the other hand to meet safety requirements and permit a quick and clean opening of the airbag. Evolution in the materials used for the production of the carrier and foam layer have not simplified this challenge, and it has been increasingly difficult to find acceptable compromises.
DE102013224996 discloses a flexible skin for an airbag cover, which comprises at least one layer of a plastic material, the layer of plastic material having particles of a further material incorporated therein and dispersed over the entire skin. The particles are made of a material with a higher melting point than the plastic material. The flexible skin is said to have an elongation at break of maximum 200% and a tensile strength of minimum 9 N/mm2. The inventors have however observed that this combination of mechanical properties cannot be reproduced with the materials disclosed in DE102013224996 as can be understood from the comparative experiments represented below.